Hello, As some of you may know, Pharo is being enhanced with support for multiple bytecode sets. This will allow to run at the same time compiled methods with 2 different bytecode sets for experimentation and to add easily the new bytecode set described in this ESUG paper <http://esug.org/data/ESUG2014/IWST/Papers/iwst2014_A%20bytecode%20set%20for%...> . The virtual machine knows which bytecode set the compiled method uses for its encoding based on the sign of its header. This means that on the contrary to right now, a compiled method *can* have a negative header. The issue is that typically operations such as #bitAnd: or #+ to access bits of the smallinteger does not work the same way on negative integers (2 complement's implementation of integers), resulting in incorrect values. I am fixing the problematic cases in Pharo. One case is in Fuel and I can't fix it myself. It's in: FLCompiledMethodCluster>>#serializeInstance: aCompiledMethodToSerialize with: anEncoder "some code" header := aCompiledMethod header. "some code" anEncoder *encodeUint32*: header "some code" This looks problematic because now header is not a uint anymore but a sint. In addition, #encodeUint32: uses #bitAnd: which may work differently on negative integers. What do you fuel experts think ? Is there anything to fix there ? Best, Clement